Can Variables Be Capital Letters?

Date: 09/19/2006 at 10:32:15
From: Annie
Subject: Naming variables

I have a friend who was marked down for using capital letters for
variables--3A.  The teacher said it must be a lower case letter--3a.
I can't find a hard and fast rule about this anywhere.  My question
is, is it a rule or is it what people usually do?

Date: 09/19/2006 at 12:28:44
From: Doctor Peterson
Subject: Re: Naming variables

Hi, Annie.

There is no rule that you can't use a capital letter as a variable; it 
is done all the time, though certainly less often than lower case.  
For example, the area of a trapezoid is sometimes written as

  A = (B + b)h/2

where A is the area, B is one base and b is the other, and h is the 
height.

There IS a rule, however, that once a variable is defined, you should 
not change the case.  Often students prefer writing capital letters, 
and will change a variable r in a problem to R in their answer.  This 
should be discouraged, as you can see from my example above: B and b 
are different things, and if you habitually write b as B, you will 
mess up the formula badly!

I would never mark an answer wrong if the student chose a capital 
letter for himself; I might call it wrong if he changed the case of a 
variable that was given in a problem, especially if there were mixed 
cases given.

If you have any further questions, feel free to write back.


- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/ 

Date: 09/23/2006 at 00:40:14
From: Annie
Subject: Thank you (Naming variables)

Thanks so much for the help.  You answered my question completely and
I really appreciate your quick response.  It is great to know I can
get my questions answered when I have them.  Thanks again!  Annie